xml-tree parser (Haskell) for graph-library

Question

I'm writing a library for working with graphs. The primary task - parsing xml-tree. The tree looks like

<graph nodes=4 arcs=5>
    <node id=1 />
    <node id=2 />
    <node id=3 />
    <node id=4 />
    <arc from=1 to=2 />
    <arc from=1 to=3 />
    <arc from=1 to=4 />
    <arc from=2 to=4 />
    <arc from=3 to=4 />
</graph>

Structure for storing:

type Id = Int

data Node = Node Id deriving (Show)
data Arc = Arc Id Id deriving (Show)

data Graph = Graph { nodes :: [Node],
             arcs  :: [Arc]}

How to write data from the xml file into this structure? I can not write a parser for xml tree of this kind (HXT library)

Answer 1

Assuming that you convert that into proper XML (surround all the attribute values with quotes), the following code will work (using xml-enumerator):

{-# LANGUAGE OverloadedStrings #-}
import Text.XML.Enumerator.Parse
import Control.Monad
import Data.Text (unpack)
import Control.Applicative

type Id = Int

data Node = Node Id deriving (Show)
data Arc = Arc Id Id deriving (Show)

data Graph = Graph { nodes :: [Node],
             arcs  :: [Arc]}
  deriving Show

main = parseFile_ "graph.xml" decodeEntities $ force "graph required" parseGraph

parseGraph = tagName "graph" getCounts $ \(nodeCount, arcCount) -> do
    nodes <- replicateM nodeCount parseNode
    arcs <- replicateM arcCount parseArc
    return $ Graph nodes arcs
  where
    requireNum name = do
        x <- requireAttr name
        case reads $ unpack x of
            (i, _):_ -> return i
            _ -> fail $ "Invalid integer: " ++ unpack x
    getCounts = do
        n <- requireNum "nodes"
        a <- requireNum "arcs"
        return (n, a)
    parseNode = force "node required" $ tagName "node"
        (Node <$> requireNum "id") return
    parseArc = force "arc required" $ tagName "arc"
        (Arc <$> requireNum "from" <*> requireNum "to") return

Outputs:

Graph {nodes = [Node 1,Node 2,Node 3,Node 4], arcs = [Arc 1 2,Arc 1 3,Arc 1 4,Arc 2 4,Arc 3 4]}

Answer 2

Do you need to use an XML library? The 'tagsoup' library might be just as effective for not-really-xml like this:

import Text.HTML.TagSoup
import Data.Maybe

main = do
    s <- readFile "A.dat"

    -- get a list of nodes and arcs
    let g' = catMaybes
                [ case n of
                    TagOpen "node" [(_,n)]        -> Just (Left  $ Node (read n)) 
                    TagOpen "arc"  [(_,n), (_,m)] -> Just (Right $ Arc (read n) (read m))
                    _ -> Nothing

                | n <- parseTags s ]

    -- collapse them into a graph
    let g = foldr (\n g -> case n of
                                Left  n -> g { nodes = n : nodes g }
                                Right a -> g { arcs  = a : arcs  g }
                        ) (Graph [] []) g'

    print g

Running this:

> main
Graph {nodes = [Node 1,Node 2,Node 3,Node 4], arcs = [Arc 1 2,Arc 1 3,Arc 1 4,Arc 2 4,Arc 3 4]}